1. Field of the Invention:
The present invention relates generally to an image forming apparatus, and more particularly to an image forming apparatus for forming an image by developing an electrostatic latent image on a latent image carrier into a visible image thereon and transferring the visible image onto a transfer sheet held on a drum-shaped or belt-shaped holder.
2. Discussion of Background:
Image forming apparatus of the type described are actually used as copying machines, color copying machines, monochromatic copying machines with an editing capability, printing machines, and various electrostatic recorders. These machines or apparatus are generally designed to use transfer sheets of plural sizes. Therefore, holders for holding these transfer sheets have to have a peripheral length capable of holding transfer sheets of the longest size.
The period of time required to transfer a visible image onto a transfer sheet is proportional to the length of the visible sheet, i.e., the length of the transfer sheet.
Where the speed of rotation of a holder is constant, the time required to transfer a visible image is determined by the longest size of transfer sheet irrespective of the length of a transfer sheet used for the image transfer. This is problematic since when a visible image is transferred several times as in a color copying machine, the efficiency of image recording is low if a shorter transfer sheet is employed. Stated otherwise, if the length of a transfer sheet is smaller, the time needed for visible image transfer is also shorter correspondingly, and an image can be formed with higher efficiency by shortening the time interval between consecutive image transfer cycles.
To achieve greater image forming efficiency, there has been proposed a method of speeding up an image forming process by accelerating and decelerating the rotation of a transfer sheet holder during an interval after one transfer cycle has been completed and before a next transfer cycle is started (see Japanese Laid-Open Patent Publication No. 60-218673).
The proposed method is effective in increasing the speed of the image forming process, but suffers various problems which should be solved before the method can be reduced into practice.
Take, for example, a color copying machine employing fixing rollers as an image fixing device. A transfer sheet onto which a number of visible images have been transferred as a combined color visible image is separated from a holder and fed into the image fixing device in which the color visible image is fixed under pressure and heat to the transfer sheet which is thereafter discharged out of the copying machine. Where the transfer sheet is long, it may happen, after the final visible image has been transferred, for the leading end of the transfer sheet carrying the colored image to be gripped and fed by the fixing rollers when the trailing end of the transfer sheet has just passed through an image transfer region within the copying machine. Since the fixing rollers normally rotate at a constant speed, if a transfer drum holding the transfer sheet as the holder were accelerated while the leading end of the transfer sheet is being gripped by the fixing rollers, then the transfer sheet would sag in front of the fixing rollers, allowing unfixed toner on the transfer sheet to be attached to neighboring members of the machine. The toner image would then be ruined.
To overcome such a problem, it would be possible to control, i.e., accelerate and decelerate, the fixing rollers in timed relation to the transfer drum. Inasmuch as the amount of heat generated by the fixing rollers per unit time remains constant, however, sufficient heat would not be applied to the transfer sheet when the fixing rollers would be rotated at a higher speed, and toner would not be well fixed. The amount of heat from the fixing rollers might be increased during rotation of the fixing rollers at a higher speed. Toner would not be well fixed, however, unless the speed control of the fixing rollers and the heat control of the same were effected in precisely timed relation.
Thus, acceleration and deceleration of the holder requires the positional relationship between the image transfer region and the image fixing device to be taken into consideration. Where a heated plate or an infrared heater which does not grip a transfer sheet is employed as the image fixing device, the transfer sheet does not sag, but the problem of irregular heat application and hence an image fixing failure remains unsolved since the speed of travel of the transfer sheet varies with acceleration and deceleration of the holder.
The positional relationship between the image transfer region and a clamp device should also be considered. The leading end of a transfer sheet is clamped by a clamp of the holder. The clamp device is disposed in a space within the image forming apparatus and positioned so as to cause the clamp to clamp the leading end of the transfer sheet. When the leading end of the transfer sheet is clamped, the clamp passes through the clamp device. If the holder as it rotates were accelerated and decelerated at this time, the clamp would fail to clamp the sheet unless the rotation of a resist roller for delivering the transfer sheet were controlled in synchronism with the holder. Such control of the rotation of the resist roller would however require a complex mechanism.
No problem will arise if acceleration and deceleration of the holder are controlled using a large-size, large-capacity motor for driving the holder. However, if a small-size, small-capacity motor were used for driving the motor in order to make the image forming apparatus smaller in size and lower in cost, acceleration and deceleration of the holder would not be controlled correctly. As a result, the holder would not rotate in timed relation to an optical scanner and a photosensitive body as a latent image carrier which rotates in synchronism with the optical scanner, resulting in an image transfer failure.
The invention disclosed in Japanese Laid-Open Patent Publication No. 60-218673 is not addressed to the various problems described above, and is disadvantageous as to operation reliability.